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Lego's website has the instructions for the Monkie Kid sets available for download and it seems like the Monkey King Warrior Mech does and doesn't have knee joints.
The whole knee area is constructed with one large balljoint at the front and two small ones in the back so while it's generally fixed in position it also means you can technically pop out the large balljoint and actually have a working knee, although a probably pretty wobbly one.